Fuel efficiency and more use of electric power are current trends in the aeronautical industry for commercial, military and utility aircraft, including unmanned air systems.
The fuel efficiency of conventional turbofan engines has improved significantly through the increase of the by-pass ratio, leading to engines of very large diameter which complicate their integration in the aircraft. The maximum power of conventional turbofan engines is generally determined by take-off or end of climb requirements and therefore the engine core is oversized for cruise.
The rapid development of more efficient and capable electric motors and batteries has prompted substantial research into the application of electric propulsion to aerospace vehicles.
Given the large variation of power required in the different phases of flight, various proposals have been made to use a combination of electric and thermal engines for take-off and climb and purely electric flight for the cruise phase, which requires substantially less power. The most common arrangement of hybrid propulsion systems is to use the thermal engine to drive an electrical generator to supplement the electrical power supplied by the batteries to the electric motors which drive fans or propellers.
U.S. Pat. No. 8,324,746 discloses a hybrid engine comprising a jet engine arranged for the transmission of torque to a rotor (whether a fan or a propeller) by means of a shaft, an electric generator arranged to be driven by said shaft and an electric motor arranged to be driven by the output of said generator for selective application of torque to the rotor and a clutch arranged in the path between the shaft and primary rotor selectively operable to mechanically disconnect the shaft from torque transmission with the rotor such that the rotor can be driven by any combination of the turbine and/or the electric motor.
U.S. Pat. No. 8,596,036 discloses a hybrid propulsive engine comprising a jet engine and a device for extracting energy at least partially in the form of electric power from the jet engine and converting at least a portion of the electric power to torque in an electric motor to drive a rotor.
A disadvantage of these hybrid engines is that they require a specific and powerful generator on the jet engine to drive the electric motor. Another undesirable characteristic is that the electric motors must be able to produce all the thrust even for the flight regimes requiring the maximum power and are, therefore, oversized for the cruise condition.